This invention relates generally to a method and apparatus for forming blown thermoplastic articles, and more particularly to a method and apparatus which compression molds an extruded parison into a preform and then expands the preform into a blown article.
Various methods for forming blown thermoplastic articles are known in the prior art. Examples of such methods are disclosed by U.S. Pat. Nos. 3,940,225, Re. 29,045, 3,754,851, 3,941,542, 4,044,872, 4,049,761, 4,115,494, 3,307,726, 3,356,242, and 3,288,898.
In general, one of the methods for forming a blown article includes injection molding a parison and then blowing that parison into the completed article. By this method, the temperature and shape of the parison can be fairly accurately regulated. However, the injection molds themselves and the high-pressure hydraulic equipment required to develop high pressures during the injection process are quite expensive. The present invention achieves an accurately formed preform shape at a desired temperature without requiring the expensive molds and tooling required by an injection molding process.
In other methods, such as shown by U.S. Pat. Nos. 3,940,225, Re. 28,045, and 3,941,542, an extruded parison is blown into a preform shape, which is then blown into the shape of the final article. The initial preform blowing step conditions the preform to a temperature which is conductive to molecular orientation during the final blowing operation. However, the initial blowing step is apparently done at a temperature which is not conducive to molecular orientation, and therefore the final blowing operation does not take advantage of all the molecular orientation which might be achieved from stretching a preform from a parison shape to the shape of the final article. The present invention compression molds the extruded parison into a preform shape having a diameter essentially the same as the extruded parison, such that the final blowing step is performed while the material is at a temperature conducive to molecular orientation. This expansion step, therefore, lends itself to achieving greater degrees of molecular orientation over the processes of the blow-and-blow operations. Moreover, in the present invention, the preform is cooled both on its interior and exterior surfaces while in contact with the mold walls and the core pin.
Further, in extrusion-blow operations, the thickness of the parison is governed by the final article. For example, when an article having a rectangular cross-section such as a milk jug is blown, the minimum thickness of the blown article is at the corners of the rectangular shape. The sections of the final article between the corners are usually unnecessarily thick, thereby using more material. The present invention enables the formation of irregular shaped preforms, such as rectangular in cross-section, to match the shape of the article to be formed by blow molding.